The present invention relates generally to projection screen apparatus, and more particularly, to a motor-driven projection screen apparatus.
While the size of the projection screen is not of the essence, these screens which are intended for use with the present invention are generally quite large, ranging in width from 5 to 14 feet or more. They are the type of screens which are frequently found in auditoriums, in school rooms or in conference rooms. They are remotely controlled electrically by a small electric motor having a rotatable shaft which is operatively coupled to a large roller. The roller is generally journaled in a casing and the casing houses both the roller and the electric motor. The screen is, of course, rolled upon the roller when the screen is moved to a closed position and unrolled from the roller when it is moved to an exposed viewing position.
The electric motors generally employed in this environment are commonly called "gear motors" and include a small electric motor and a reduction gear mechanism coupled to the output shaft of the motor to regulate the rate of rotation of the reduction gear output shaft. The gear motors have to be small in size because of the space limitations and yet they must have sufficient drive to turn the roller which carries the heavy suspended screen. Because the screen is very large and heavy, there is considerable gravitational force tending to unwind the screen from the roller. In order to prevent the screen from peeling from the roller and thereby causing the roller and the shaft of the motor to rotate, the motor must have a braking mechanism which will hold the roller against rotation to prevent unintended movement of the screen whenever the screen is properly positioned. In order to overcome the extreme forces tending to unwind the screen, the braking mechanism must be both reliable and effective. If the braking mechanism should fail to hold the screen in its intended position, the screen will unwind and ultimately be stripped from its adhered position on the roller.
In the prior art, motors are often provided with brake mechanisms. These motors have axially movable shafts, the shafts carrying a braking surface within the motor housing which normally is urged into engagement with a braking surface on an internal axial wall of the motor housing. When the motor is operated, the braking surfaces are urged apart to release the motor as the shaft-mounted rotor axially aligns itself in the magnetic field generated by the stator. However, because of the size of the motor, the size of the braking surfaces was limited and, therefore, the holding power of the braking mechanism.
Attempts have been made to increase the efficiency of the brake by increasing the surface areas involved. In one such mechanism, the cooperating braking surfaces were formed in a conical shape. However, the capabilities of the brakes were still limited by the size of the motor, or more particularly, by the size of motor housing interior.